I have a 1986 ex France Telecom R4. Had it for 15 yrs, and it now has 150km on the clock. It recently started playing up for the first time. Seems to drop a cylinder or 2, bad vibration when idling, but works fine once one is bundling along nicely.... I,ve changed plugs, plug leads, distributor head and rotor, all to no avail. Checked compression, thats fine too. I think it must be the carb, about which i am fairly clueless. Mine is a Zenith 921F, and it doesn't feature in the Haynes manual, as probably was only fitted for sales in France. i have a feeling i need to play with the mixture scres, but I have no idea which one it is.... Can anyone help. By the by, I'm in France